1. Field of Invention
This invention is directed to absolute distance measuring devices. In particular, this invention is directed to an absolute distance measuring device that uses wavelength-dependent external cavity feedback control of a laser diode.
2. Description of Related Art
Tunable external cavity diode lasers (ECDLs) are widely used in light-based test and measurement equipment and are increasingly used in wavelength division multiplexed (WDM) optical voice and data communications devices. The tuning is accomplished by a combination of filtering the wavelength and varying the optical path length between the two end reflectors that define the extent of the external cavity. The external cavity is formed by an external feedback reflector and the facet of the laser chip that is distal with respect to the external reflector.
Diode lasers have also been used as interferometers by allowing light reflected and backscattered by a measurement target surface to re-enter the laser diode such that the wavelength and amplitude of the laser diode output is modulated as the distance to the measurement target surface is varied. In this situation, the laser is operated in a self-mixing regime, wherein the feedback from reflections off the external measurement target surface interferes with the operation of the laser diode facet reflections that ordinarily occur within the solitary laser diode. This is frequently referred to as self-mixing. The resulting signal becomes amplified by the laser medium and provides a signal that varies periodically with changes in the length of the external cavity.
Absolute distance measurement systems have been made based on the self-mixing principle, by either applying a modulation to the laser diode wavelength or by applying a modulation to the distance from the laser diode to the external measurement target surface, and analyzing the relationship between the self-mixing signal and the applied modulation to determine the absolute distance measurement.